Same as "Just because", or rather similar to "Fuck you, that's why!" when used as a response
"Just doin' it for the fuck of it, I don't care if I win this competition"
"Why are you upgrading your PC again? Is it slow already?" - "Nah bro, just for the fuck of it. The upgrade bug is biting me in the ass again"
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"